Field of the Invention
The present invention relates to an image display apparatus and a control method thereof.
Description of the Related Art
Recently improvements in the visibility of display images (images displayed on a screen) are demanded for image display apparatuses. Specifically, an increase in the ratio between the bright part and the dark part (contrast ratio) of display images is demanded for image display apparatuses.
A prior art on the image display apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. 2010-107535 and Japanese Patent Application Laid-Open No. 2008-122536.
Japanese Patent Application Laid-Open No. 2010-107535 discloses a liquid crystal display apparatus having a liquid crystal panel and a backlight unit. In the liquid crystal display apparatus disclosed in Japanese Patent Application Laid-Open No. 2010-107535, the transmittance of the liquid crystal panel in the bright part display region and the emission brightness of the backlight unit in the bright part display region are increased. Further, the transmittance of the liquid crystal panel in the dark part display region and the emission brightness of the backlight unit in the dark part display region are decreased. The bright part display region is a region where bright parts of the image are displayed, out of the regions on the screen, and the dark part display region is a region where dark parts of the image are displayed, out of the regions on the screen. A control to partially change the emission brightness of the backlight unit is called “local dimming control”.
However, the minimum size of a region where the emission brightness can be changed by the local dimming control is larger than the size of the liquid crystal element of the liquid crystal panel. Therefore, if an image having many high frequency components (e.g. an image having many fine edges) is displayed, the contrast ratio of the display image is not improved very much even if the local dimming control is performed.
Japanese Patent Application Laid-Open No. 2008-122536 discloses a liquid crystal display apparatus having a color liquid crystal panel, a monochrome liquid crystal panel, and a backlight unit. The backlight unit emits light at a predetermined emission brightness. The monochrome liquid crystal panel is disposed between the color liquid crystal panel and the backlight unit, and the light emitted from the backlight unit transmits through the monochrome liquid crystal panel, and then transmits through the color liquid crystal panel. In the liquid crystal display apparatus disclosed in Japanese Patent Application Laid-Open No. 2008-122536, not only the transmittance of the color liquid crystal panel, but also the transmittance of the monochrome liquid crystal panel is controlled. Thereby the contrast ratio of the display images improves. Such a structure of having the two liquid crystal panels is hereafter called “double panel structure”.
The minimal size of the region where the transmittance of the monochrome liquid crystal panel can be changed is the size of the liquid crystal element of the monochrome liquid crystal panel, and is smaller than the minimal size of the region where the emission brightness can be changed by the local dimming control. Therefore in the image display apparatus having the double panel structure, the contrast ratio of the display image can be easily improved, even if the display image includes many high frequency components.
However, in the case of a conventional liquid crystal di splay apparatus having the double panel structure, the emission brightness of the backlight unit is controlled to a higher value than the case of using one liquid crystal panel considering that the light emitted from the backlight unit transmits through the two liquid crystal panels. As a result, in the conventional liquid crystal display apparatus, total power consumption of the liquid crystal apparatus is increased by the use of the double panel structure.